Producing and transmitting electromagnetic waves



March 27, 1945. J, lsK 2,372,193

PRODUCING AND TRANSMITTING ELECTROMAGNETIC WAVES Filed June 5. 1940 29FW III I 25 V l 45 I4 -44\ L J 38 .49 so l8 I6 42 2a- 20 1 L] 2, A2243/, 39 /4a 25 i v /9 4a I as M n l l i LJ N V E N TOR J. B. F/SK A TTORA/ES Patented Mar. 27, 1945,

PRODUCING AND TRANSMITTING ELECTROMAGNETIC WAVES James B. Fisk, NewYork, N. Y., assignor to Bell Telephone Laboratories, Incorporated, NewYork, N. Y., a corporation of New York Application June 5. 1940, SerialNo. 338,949

6 Claims. (Oi. 315-6) This invention relates to wave transmissionsysterns "and more particularly to systems for producing and controllingvery high frequency waves.

An object of the invention is to increase the compactness and efilciencyof high frequency wave sources in the decimeter wave-length range.

Another object of the invention is to improve the effectiveness of theenergy transfer coupling between a high frequency wave source and a wavetransmission system.

A further object of the invention is to enable modulation of highfrequency waves in accordance with signals or other control forces.

An additional object of the invention is to provide simple and compactapparatus for producing harmonics of very short waves.

The principles of the invention may be embodied in very divergentsystems and structures. In one embod ment an electron gun is used toproduce a beam which is thereafter velocity varied and caused to becomebunched" or density varied in a drift space before yielding energy to anenergy-extracting field. The energy-extracting field is regenerativelycoupled to the velocity varying element to cause the apparatus toproduce continuou oscillations the frequency of which is determined bythe resonance characteristics of closed chambers or wave guide sectionswhich constitute the electron velocity varying and energy extractionelements. Only suflflcient ener y to sustain the oscillation operationis extracted from the density-varied beam and fed back to the velocityvarying device. At some point between the drift space and the feedbackener y extractor, or at a point beyond the extractor, additional energyis withdrawn from the densityvaried beam \by a resonance chamber theheld of which is in such phase as to retard the beam and thus to extractenergy from it. This latter chamber, which may serve to supply the loador transmission path, is accordingly wholly uncoupled to the oscillatorproper except through the electron beam itself, and so presents noreaction to the oscillator. For controlling the energy supplied to theload a signal operated diaphragm or piston may vary the resonancecharacteristics of the two chambers of the oscillator proper or of theload coupling chamber. Variation of the former results in frequencymodulation and or the latter in amplitude modulation of the waveimpressed upon the wave transmission path.

Other objects and aspects of the invention w.-ll be apparent from aconsideration of the following specification and appended claims takenin lustratlng various features and embodiments of the invention.

Fi 1 illustrates one embodiment of a density varied electron streamoscillator and load coupling in accordance with the invention;

Fig. 2 is a modification of the system of Fig. 1:

Fig. 3 is an embodiment of an alternative form of the system of Fig. 2,in which the load couphng chamber is placed intermediate the input andoutput chambers of the-oscillators Fig. 4 is a partial sectional viewalong the line 4-4 of Fig. 3 looking in the direction of the arrows;

Fig. 5 illustrates another modification of the invention in which theload circuit is apertured coupled to the oscillator;

Fig. 6 is a partial sectional view along the plane 6-4 of Fig. 5 lookingin the direction of the arrows, and

Fig. 7 illustrates a modification of the system of Fig. 5.

Referring to Fig. 1, an oscillator and load coupling system comprises anelectron discharge device l0 having an electron gun il, a resonant inputchamber H, a drift space 13 and a resonant output chamber ll coupled infeedback relation to the input chamber 12 by a short coaxial sec tionI5. As so far described, the structure is ubstantially identical withthat of Fig. 5 of an article by Varian and Varian at page 325 of theJournal of Applied Physics, vol. 10, No. 5. May 1939. The electrondischarge device l0 consists primarily of an envelope of glass or otherinsulating material united in any desired manner with the conductivestructure which has been described. 'Passing transversely through thelower portion of the non-conducting envelope In is a wave guide It. Thewave guide may be rectangular or of any other desired section but isillustrated in Fig. 1 as of the conventional circular or cylindricaltype. The terminal portion II which comprises a hollow resonance chamberl'l separated from the main structure of the guide by conducting septumit, the central portion ll of which is apertured to provide anelectromagnetic coupling with the remainder of the guide.

The portion i9 is preferably closed with an insulating closure such asglass or mica. At the central portion of the chamber I! are inwardlydirected protuberances 20 and 2|, the open ends of which are separatedby a short gap 22. The

end of the chamber remote from the septum II is closed by a movablediaphragm or piston 23 which maintains the structure sealed againstenconnection with the. accompanying drawing 11- trance of atmosphere buti movable under the influence of incident sound waves to vary the reso=nant characteristics of the chamber. Beyond the load coupling chamber lland in line with the protuberances 28 and 25 is a collecting anode 2%which is externally connected electrically with the cathode it of theelectron gun by a path in= cluding a source 25 of electromotive force.The cathode 28 of the electron gun is connected with the chamber if by asource 25 of focussing and accelerating electromotive force.

The chamber ll has been shown and described as a terminal portion of awave guide. It may, however, be instead a resonant cavity coupled to acoaxial line or other transmission system in any desired manner.

There may be connected between the chamber is and the chamber it asource at of potential for further accelerating the electrons. The purpose of the source 3b is to accelerate the .bunches" of electronsbetween gap 32 and gap 22 in order to reduce their time of flightbetween these two gaps. Thus little or no further buncln ing will takeplace and space charge forces will have little time to act in"debunching" the groups. The source of potential 25 may be such that theanode 24 is negative with respect to the chamber ii. In this eventelectrons will be col= lected at less than their maximum energy.

In operation, an electron stream projected from the electron gun passesthrough the gap at between the central perforated screens of theresonance chamber 112 and its electrons are sublected by the highfrequency field at the gap to a velocity variation. In their furthercourse through the drift space 93, decelerated electrons fall behind andaccelerated electrons gain on the average velocity or non-acceleratedelectrons with the result that density variation occurs in the electronstream as it arrivesat the gap 32 of the resonance chamber it. With thephase of the electromagnetic field in the gap 32 such as to oppose andslightly retard the electrons, energy is extracted from the beam by theresonance chamber l4 and sufficient energy is fed back by the coaxialconnector l5 to the resonance chamber l2 to maintain the operation ofoscillation production. The density-varied electron stream aftertraversing the gap 32 is prolected through the gap 22 of the loadcoupling chamber ll. Preferably, the gap 22 is so placed with referenceto the gap 32 and the amount of energy extraction by the gap 32 is sosmall that the stream reaching 22 is still density varied. The resonancechamber I1 is designed to build up a strong electromagnetic field inorder to extract the greatest possible amount of energy from theelectron stream. The electrons retarded after passing through the gap 22proceed to a collecting anode 24 where they are absorbed and withdrawnfrom the discharge device. It will, therefore, be apparent that with theresonant chambers l2 and I4 and their feedback l5 so designed as tomaintain continuous oscillating action and to produce density variationof the electron stream with a minimum of energy dissipation in thechambers l2 and It, the principal part of the energy of a high velocitystream is available for utilization at the gap 22. Moreover, the chamberI1 is effectively self-shielded from reaction in any way upon thechamber l2 or chamber I4 so that there is no electromagnetic couplingbetween these chambers and the resonance chamber I'I except theextremely small coupling along the electron stream itself. It is,therefore, possible to operate the system as a whole with varying loadcharacteristics in the wave guide l6 and even to vary the resonancecharacteristics of the chamber H to a considerable degree without anyappreciable effect upon either the frequency or the intensity of the density variations of the electron beam. This is con= ducive to highstability as to both frequency and amplitude in the resultingoscillation energy available.

Advantage may be taken of the self-shielding feature of the loadcoupling to use the resonance chamber ll for modulation purposes. Speechor other sound waves directly or indirectly impressed upon the diaphragm23 serve to throw the chamber ll out of resonance with its normaloscillation frequency and that of the oscillations produced thus causingthe energy supplied through the aperture 19 to the wave guide system tobe controlled or varied in accordance with the sound waves. The resonantchamber ll when used for amplitude modulation in the manner which hasjust been described is normally tuned either above or below thefrequency of the oscillations produced by an amount such that anexcursion of the diaphragm 23 in one direction brings the resonancefrequency of the chamber i'l toward the frequency of the oscillatorwhile an excursion in the other direction causes the resonance frequencychamber ill to recede from the frequency of the oscillator. Theresonance characteristic of such a chamber may be made very steep andthe modulating operation may cause the chamber to operate over asubstantially linear portion of its resonance characteristic.

It is also possible to utilize a system of the type illustrated in Fig.l for the production and transmission to the wave guide it ofoscillations which are harmonics of the natural resonance fre quenciesof chambers l2 and H. For this purpose, the resonance chamber I! may bedesigned to have a natural frequency which is substan tially a harmonicof that of chambers l2 and M. The operation in the production ofoscillations and transfer of electromagnetic wave energy to theresonance chamber II will be identical with that which has beendescribed except that the density variations or pulses of electronsreaching the gap 22 will occur at intervals of a number of cycles ratherthan once each cycle. Where these harmonic frequency oscillations are tobe modulated, the normal tuning of the resonance chamber ll will departslightly from the true harmonic frequency in order to permit modulatingopera= tion on the substantially linear portion of one side of theresonance characteristic. The wave guide it will, of course, be designedfor effective transmission of the oscillations produced in the chamberll.

The aperture I9 is preferably made such as to enable an effective loosecoupling between the resonance chamber l1 and the remainder of the waveguide l6 whereby substantially all the energy of the electromagneticfield within the resonance chamber l1 except that dissipated internallyi passed on to the wave guide.

The apparatus of Fig. 2 involves an electron discharge device consistingof cup-shaped portions 34 and 35 of dielectric material sealed to aconducting structure 36 involving three resonant chambers 31, 38 and 39,preferably tuned to substantially the same frequency. An electron gun 40within the end portion 34 projects electrons through gap ll of chamber31, gap 42 of chamber 38 and gap 43 of chamber 39. Between the gaps 4|and 42 is a drift tube 50 of such length as to permit the electronswhich are velocity varied at gap 4| to rearrange their relativepositions so that the electron stream .as a whole is density varied atthe gap 42. Oscillating action is maintained by a small aperturecoupling through aperture-44 between the chambers 31 and 38. Thus, asmall amount of oscillating energy is fed back to the chamber 31. Thedensity-varied electron beam traversing gap 43 yields energy to the loadcoupling chamber 39 which constitutes a terminal portion of the waveguide 45 and is associated therewith by an energy transfer aperture 46sealed in the usual manner to exclude atmosphere from the electrondischarge device. If desired, an adjustable conductive iris may bepositioned over the outside" of the aperture 46 to enable adjustment ofthe coupling between chamber 39 and the remainder of the wave guide 45to the most efiicient value.

After extraction of a considerable portion of its energy by the chamber39, the electron beam modulation may be effected by manipulation bysound waves or otherwise of the position of the diaphragm or piston 48constituting an end wall of the resonant chamber 39. The chambers 31, 38and 39 in section perpendicular to the drawing of Fig. 2 may be madecircular or rectangular, as desired. Tne wave guide 45 may be tapered asat 49 in order to permit effective coupling to the chamber 39.

Fig. 3 illustrates another modification in which the resonant oscillatorchambers and 52 contitute respectively, the upper and lowerhemicylindrical portions of a conductive contamer 53, the centralportion of which receives the terminal resonance chamber 54 of acircular wave guide 55. The chambers 5| and 52 contain the modulatinggap 56 and the energy-extracting gap 51, respectively. The load couplingchamber 54 contains the energy-extracting gap 58. This chamber iscoupled by aperture 59of glass or other non-conducting material to theremainder of the wave guide and the opposite end of the chamber isclosed by the modulating diaphragm 60. In this structure the electronbeam from the cathode gun undergoes an electron velocity varyingoperation at gap 56 and thereafter passes along the drift tube 6| to thegap 58. It has then become density varied and is in condition to yield alarge portion of its energy to the energy-extracting chamber 54. Theelectrons retarded at that point pass on to the gap 51 where the chamber52 extracts suflicient energy to feed back a sustaining force to thechamber 5| by way of the coupling aperture 62 in the conductive baflleor septum 63, shown in Fig. 4.

In Fig. 5, the resonant input chamber 64 and the resonant output chamber65 of the oscillator are provided with a common end wall or diaphragm 66which may be controlled in accordance with sound waves or signal orcontrol forces to vary the resonance characteristics of the chambers 64and 65 simultaneously. Since these resonance chambers determine thefrequency of the oscillations produced and supplied to the wave guide 61through aperture 68, it will be apparent that this device producesfrequency modulation of the electromagnetic waves. In this structure theresonant output chamber of the oscillator and the resonant load couplingcham-. ber of the wave guide are combined. It will be cordance with thearrangement of Fig.2, thus permitting the load system to be effectivelyisolated from the oscillation-producing systemfrom the standpoint ofelectromagnetic reaction of the load upon the oscillator. Where thethree separate chambers are employed frequency modulation may beeffected by causing the common diaphragm or end wall of all threechambers to be displaced in accordance with the modulating sound wave orother controlling force.

As indicated in Fig. 6, the chambers 64 and 65 may comprise the upperand lower hemicyclindrical portions of the cylindrical container 63 andthe wave guide 61 may be tapered as at 10 in order to suitably match thechamber 65 in impedance and in its effective frequency transfercharacteristics.

Fig. '7 illustrates diagrammatically a modification of the structure ofFig. 5 which simplifies the mechanical construction problems involved inconnection with very small resonance chambers where a considerable driftis required to provide density variation. In this structure theresonance chambers 1| and 12 between which is the drift tube 15 areprovided with individual end walls or diaphragms 13 and 14 driven by acommon mechanical structure 15 actuated by sound waves or other signalor control forces to vary the frequency of the oscillations produced inaccordance with signals as in the structure of Fig. 5.

Although the resonance chambers and drift spaces have mostly been shownwith circular symmetry about a central beam of electrons it will beunderstood that the beam may be broadened to pass from the electron gunto the final collector'as a thin sheet of electrons. The systems of Fig.2, 5 or '1 may readily be so modified by so designing the electron gunthat its muzzle is broad in the direction perpendicular to the paper. Itwill be readily understood that the gaps 4 I, 42 and 43 and the drifttubes will likewise be broadened in that same direction as will also theanode 41. In structures of that type, the resonance chambers 31, 38, 33and the insulating cap portions 34 and 35 will likewise be elongated inthe same direction. However the resonant cavities themselves may have avariety of shapes *and they do not necessarily possess complete symmetrywith respect to the electron beam.

It will also be understood that all coupling apertures such as 68 ofFig. 5 may be atmospherically sealed by dielectric material and that ineach of the systems illustrated energizing and other external circuitscorresponding to those illustrated in Fig. 1 may be provided.

What is claimed is:

1. A system for producing and transmitting oscillations comprising anelectron stream emitter, a resonant electrical device, the field ofwhich is in the path of the electron stream to velocity vary theelectrons, a second resonant electrical device, the field of which islikewisein the path of the stream at a point at which the stream isdensity-varied to enable the second device to extract energy from thestream, means coupling the devices to enable oscillations of the seconddeviceto be fed back to the first device, thus constituting apparatusfor producing continuous oscillations, a third resonant device whollyuncoupled to the continuous oscillation-producing apparatus except thatits field also intercepts the path of the electron stream at a pointwhere denity variation exists to enable it to extract energy i'rom thedensity-varied stream, means electrically biasing the third resonantdevice to a positive potential with respect to the second device toincrease the velocities of electrons intermediate the points of thesecond and third resonant devices at which the electron stream isdensityvaried in order to reduce the transit time therebetween, and anoutput load transmission system to which oscillations are to *besupplied electrically connected to the third resonant device wherebyreaction of the output load transmission system upon theoscillation-producing apparatus is precluded.

2'. In combination, a wave guide, means bounding a portion of the waveguide in such manner as to constitute a resonant electrical chamberdirectly coupled to the remainder of the guide, two external resonancechambers having frequency characteristics similar to that of the first,and means for impelling a stream of electrons through a portion of thefields of each of the three chambers, the separation of the positions ofwhich the stream encounters the fields being such that velocityvariation of the electrons passing through the first chamber results insubstantial density variation of the stream at the points where it isexposed to the fields of the other two.

3. The combination of claim 2 in which the of the second chamber mayextract energy thered rom, additional coupling means between theinterior of the second chamber and the interior of the first chamberfeeding back energy from the second chamber to the first wherebycontinuous oscillations of the natural resonance of the chambers areproduced, the third chamber being wholly first two chambers areelectrically coupled to permit feedback from the second to the first ofenspaced openings, the openings of the chambers being in alignmentwhereby electrons may be projected'through the chambers, means forprojecting an electron stream through the chambers in sequence, theseparation of the exit opening of the first chamber from the entranceopening of the second chamber being such that electrons velocity variedby interaction with an internal electric field of the first chamber aredensity varled at the position in which they enter the second chamber,whereby an internal electric field uncoupled from the oscillationproducing means except through the electron stream and having the gapbetween its openings positioned at a point in the stream ofsubstantially maximum density variation, whereby an electric fieldwithin the third chamber is able to extract useful energy from theelectron stream with negligible reaction upon the oscillation producingmeans, electrical biasing means connected between the second and thirdchambers to bring the third chamber to a potential considerably morepositive than that of the second chamber and an output transmissionsystem electrically connected to the interior of the third chamber towithdraw oscillation en ergy therefrom and otherwise uncoupled with theoscillation producing means.

6. In an electrical discharge device, an electron gun for projecting anelectron stream, three substantially closed electrically conductingresonance chambers having gaps aligned with the stream to permit itspassage through the electromagnetic fields of the chambers, the chambersbeing selfshielding to eliminate direct coupling between their fields ordirect reaction upon the electron stream except in the region of theirrespective gaps, means whereby the first gap traversed introduces anelectron velocity variation upon the stream, an external feedback to thechamber of the first-traversed gap from one of the other chambers toreinforce the electromagnetic field therein and to enhance the electrondensity varying operation whereby continuous oscillations ensue, anoutput energy utilization path connected to the remaining chamber towithdraw energy therefrom, the distances of the gaps of the other twochambers from the first gap and the velocities of the electrons being sorelated that the steam exhibits an effective electron density variationat their gaps whereby energy'may be extracted from the stream by each ofsaid two chambers, and means for considerably increasing the velocitiesof all electrons by a fixed amount intermediate the second and thirdgaps in order to reduce the transit time therebetween and to minimizeany tendency toward decrease of the electron density variation of thestream.

JAMES B. FISK.

